Pyrolysis processes, such as steam cracking, can be utilized for converting saturated hydrocarbons to higher-value products such as light olefins, e.g., ethylene and propylene. Besides these useful products, the pyrolysis of hydrocarbons can also produce a significant amount of undesirable, relatively low-value products, such as pyrolysis tar, e.g., steam-cracker tar (“SCT”).
SCT is a high-boiling, viscous, reactive material comprising complex, ringed and branched molecules that can polymerize and foul equipment. SCT also contains high molecular weight non-volatile components including paraffin-insoluble compounds, such as pentane-insoluble (“PI”) compounds and heptane-insoluble (“HI”) compounds. The high molecular weight compounds are typically multi-ring structures that are also referred to as tar heavies (“TH”). These high molecular weight molecules can be generated during the steam cracking process, and their high molecular weight leads to high viscosity which limits desirable SCT disposition options. For example, it is desirable to find higher-value uses for SCT, such as for fluxing with heavy hydrocarbons, especially heavy hydrocarbons of relatively high viscosity. It is also desirable to be able to blend SCT with one or more heavy oils, examples of which include bunker fuel, burner oil, heavy fuel oil (e.g., No. 5 or No. 6 fuel oil), high-sulfur fuel oil, low-sulfur oil, regular-sulfur fuel oil (“RSFO”), and the like.
One difficulty encountered when blending heavy hydrocarbons is fouling that results from precipitation of high molecular weight molecules, such as asphaltenes. See, e.g., U.S. Pat. No. 5,871,634, which is incorporated herein by reference in it's entirely. In order to mitigate asphaltene precipitation, an Insolubility Number, IN, and a Solvent Blend Number, SBN, are determined for each blend component. Successful blending is accomplished with little or substantially no precipitation by combining the components in order of decreasing SBN, so that the SBN of the blend is greater than the IN of any component of the blend.
Attempts at neat SCT hydroconversion to reduce viscosity and to improve both IN and SBN, have not led to a commercializable process, primarily because fouling of process equipment could not be substantially mitigated. For example, neat SCT hydroconversion results in rapid catalyst coking when the hydroconversion is carried out at a temperature in the range of about 250° C. to 380° C., a pressure in the range of about 5400 kPa to 20,500 kPa, using a conventional hydroconversion catalyst containing one or more of Co, Ni, or Mo. This coking has been attributed to the presence of TH in the SCT. Although catalyst coking can be reduced by increasing hydrogen partial pressure, reducing space velocity, and operating at a lower temperature, SCT hydroconversion under such conditions is undesirable because increasing hydrogen partial pressures worsens process economics owing to increased hydrogen and equipment costs. Also, because of the increased hydrogen partial pressure, reduced space velocity, and reduced temperature range, an unacceptable level of undesired hydrogenation reactions can occur, leading to precipitation of the higher IN molecules.
Previous hydroconversion options using conventional hydroconversion process conditions and catalysts faced at least two major obstacles to commercialization. First, high-molecular weight SCT components, especially those having high-viscosity, low SBN and high IN, can adsorb onto the catalyst surfaces. This led to excessive coking on catalyst, which by way of even more hydrogen starvation of aromatic molecules, resulted in poorer solubility of these molecules, eventually ending in process shutdown. Second, because of high hydrogen cost, aromatic ring saturation needed to be limited to prevent poor process economics.
One approach taken to overcome these difficulties is disclosed in International Patent Publication No. 2013/033580, which is incorporated herein by reference in its entirety. The application discloses hydroconverting SCT in the presence of a utility fluid comprising a significant amount of single and multi-ring aromatics. The hydroconverted tar product generally has a decreased viscosity, decreased atmospheric boiling point range, and increased hydrogen content over that of the SCT chargestock, resulting in improved compatibility with fuel oil and blend-stocks. The reference discloses a utility fluid having an ASTM D86 10% distillation point ≥60° C. and a 90% distillation point≤360° C. The amounts of utility fluid and SCT are in the range of from about 20.0 wt. % to about 95.0 wt. % of SCT and from about 5.0 wt. % to about 80.0 wt. % of utility fluid. Hydroprocessing conditions include a temperature in the range of about 50° C. to 500° C., an LHSV of the combined utility fluid/SCT in the range of about 0.1 h−1 to 30 h−1, a molecular hydrogen partial pressure in the range of about 0.1 MPa to 8 MPa, and a molecular hydrogen consumption rate of about 53 S m3/m3 to about 445 S m3/m3 based on the volume of SCT.
Although attempts have been made to develop a commercializable process for converting SCT to lower boiling more valued products, they have fallen short of this goal. Further improvements are therefore desired, e.g., improvements in decreasing the amount of catalyst required without significantly increasing process severity and/or decreasing the amount of utility fluid needed.